TY - JOUR T1 - Thinking outside the channel: modeling nitrogen cycling in networked river ecosystems JF - Frontiers in Ecology and the Environment Y1 - 2011 A1 - Helton, A.M. A1 - Poole, G.C. A1 - Meyer, J.L. A1 - Wollheim, W.M. A1 - Peterson, B.J. A1 - Mulholland, P.J. A1 - Bernhardt, E.S. A1 - Stanford, J.A. A1 - Arango, C.P. A1 - L.R. Ashkenas A1 - Cooper, L.W. A1 - W. K. Dodds A1 - Gregory, S.V. A1 - O’Hall, R. A1 - Hamilton, S.K. A1 - Johnson, S.L. A1 - W.H. McDowell A1 - Potter, J.D. A1 - Tank, J.L. A1 - Thomas, S.M. A1 - Valett, H.M. A1 - Webster, J.R. A1 - Lydia H. Zeglin AB -

Agricultural and urban development alters nitrogen and other biogeochemical cycles in rivers worldwide. Because such biogeochemical processes cannot be measured empirically across whole river networks, simulation models are critical tools for understanding river-network biogeochemistry. However, limitations inherent in current models restrict our ability to simulate biogeochemical dynamics among diverse river networks. We illustrate these limitations using a river-network model to scale up in situ measures of nitrogen cycling in eight catchments spanning various geophysical and land-use conditions. Our model results provide evidence that catchment characteristics typically excluded from models may control river-network biogeochemistry. Based on our findings, we identify important components of a revised strategy for simulating biogeochemical dynamics in river networks, including approaches to modeling terrestrial–aquatic linkages, hydrologic exchanges between the channel, floodplain/riparian complex, and subsurface waters, and interactions between coupled biogeochemical cycles.

VL - 9 UR - https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/080211 ER - TY - JOUR T1 - Inter-regional comparison of land-use effects on stream metabolism JF - Freshwater Biology Y1 - 2010 A1 - Bernot, M.J. A1 - Sobota, D.J. A1 - Hall, R.O. A1 - Mulholland, P.J. A1 - W. K. Dodds A1 - Webster, J.R. A1 - Tank, J.L. A1 - L.R. Ashkenas A1 - Cooper, L.W. A1 - Dahm, C.N. A1 - Gregory, S.V. A1 - Grimm, N.B. A1 - Hamilton, S.K. A1 - Johnson, S.L. A1 - W.H. McDowell A1 - Meyer, J.L. A1 - Peterson, B. A1 - Poole, G.C. A1 - Valett, H.M. A1 - Arango, C.P. A1 - Beaulieu, J.J. A1 - Burgin, A.J. A1 - Crenshaw, C. A1 - Helton, A.M. A1 - Johnson, L. A1 - Merriam, J. A1 - Niederlehner, B.R. A1 - O'Brien, J.M. A1 - Potter, J.D. A1 - Sheibley, R.W. A1 - Thomas, S.M. A1 - Wilson, K. AB - 1. Rates of whole-system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first-order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions. 2. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban-influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input. 3. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams. 4. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike’s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land-use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land-use categories in the catchment as regulators of ER. 5. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land-use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER. 6. Overall, consideration of the data separated by land-use categories showed reduced inter-regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism. VL - 55 ER - TY - JOUR T1 - Nitrate removal in stream ecosystems measured by 15N addition experiments: Total uptake JF - Limnology and Oceanography Y1 - 2009 A1 - Hall, R.O. Jr. A1 - Tank, J.L. A1 - Sobota, D.J. A1 - Mulholland, P.J. A1 - O'Brien, J.M. A1 - W. K. Dodds A1 - Webster, J.R. A1 - Valett, H.M. A1 - Poole, G.C. A1 - Peterson, B.J. A1 - Meyer, J.L. A1 - W.H. McDowell A1 - Johnson, S.L. A1 - Hamilton, S.K. A1 - Grimm, N.B. A1 - Gregory, S.V. A1 - Dahm, C.N. A1 - Cooper, L.W. A1 - L.R. Ashkenas A1 - Thomas, S.M. A1 - Sheibley, R.W. A1 - Potter, J.D. A1 - Niederlehner, B.R. A1 - Johnson, L.T. A1 - Helton, A.M. A1 - Crenshaw, C.M. A1 - Burgin, A.J. A1 - Bernot, M.J. A1 - Beaulieu, J.J. A1 - Arango, C.P. AB -

We measured uptake length of 15NO3− in 72 streams in eight regions across the United States and Puerto Rico to develop quantitative predictive models on controls of NO3− uptake length. As part of the Lotic Intersite Nitrogen eXperiment II project, we chose nine streams in each region corresponding to natural (reference), suburban-urban, and agricultural land uses. Study streams spanned a range of human land use to maximize variation in NO3− concentration, geomorphology, and metabolism. We tested a causal model predicting controls on NO3− uptake length using structural equation modeling. The model included concomitant measurements of ecosystem metabolism, hydraulic parameters, and nitrogen concentration. We compared this structural equation model to multiple regression models which included additional biotic, catchment, and riparian variables. The structural equation model explained 79% of the variation in log uptake length (SWtot). Uptake length increased with specific discharge (Q/w) and increasing NO3− concentrations, showing a loss in removal efficiency in streams with high NO3− concentration. Uptake lengths shortened with increasing gross primary production, suggesting autotrophic assimilation dominated NO3− removal. The fraction of catchment area as agriculture and suburban-urban land use weakly predicted NO3− uptake in bivariate regression, and did improve prediction in a set of multiple regression models. Adding land use to the structural equation model showed that land use indirectly affected NO3− uptake lengths via directly increasing both gross primary production and NO3− concentration. Gross primary production shortened SWtot, while increasing NO3− lengthened SWtot resulting in no net effect of land use on NO3− removal.

VL - 54 UR - https://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.2009.54.3.0653 ER - TY - JOUR T1 - Nitrate removal in stream ecosystems measured by 15N addition experiments: Denitrification JF - Limnology and Oceanography Y1 - 2009 A1 - Mulholland, P.J. A1 - Hall, R.O. A1 - Sobota, D.J. A1 - W. K. Dodds A1 - Findlay, S.E.G. A1 - Grimm, N.B. A1 - Hamilton, S.K. A1 - W.H. McDowell A1 - O'Brien, J.M. A1 - Tank, J.L. A1 - L.R. Ashkenas A1 - Cooper, L.W. A1 - Dahm, C.N. A1 - Gregory, S.V. A1 - Johnson, S.L. A1 - Meyer, J.L. A1 - Peterson, B.J. A1 - Poole, G.C. A1 - Valett, H.M. A1 - Webster, J.R. A1 - Arango, C.P. A1 - Beaulieu, J.J. A1 - Bernot, M.J. A1 - Burgin, A.J. A1 - Crenshaw, C.L. A1 - Helton, A.M. A1 - Johnson, L.T. A1 - Niederlehner, B.R. A1 - Potter, J.D. A1 - Sheibley, R.W. A1 - Thomas, S.M. AB -

We measured denitrification rates using a field 15NO3− tracer-addition approach in a large, cross-site study of nitrate uptake in reference, agricultural, and suburban-urban streams. We measured denitrification rates in 49 of 72 streams studied. Uptake length due to denitrification (SWdenn) ranged from 89 m to 184 km (median of 9050 m) and there were no significant differences among regions or land-use categories, likely because of the wide range of conditions within each region and land use. N2 production rates far exceeded N2O production rates in all streams. The fraction of total NO3− removal from water due to denitrification ranged from 0.5% to 100% among streams (median of 16%), and was related to NH4+ concentration and ecosystem respiration rate (ER). Multivariate approaches showed that the most important factors controlling SWden were specific discharge (discharge / width) and NO3− concentration (positive effects), and ER and transient storage zones (negative effects). The relationship between areal denitrification rate (Uden) and NO3− concentration indicated a partial saturation effect. A power function with an exponent of 0.5 described this relationship better than a Michaelis-Menten equation. Although Uden increased with increasing NO3− concentration, the efficiency of NO3− removal from water via denitrification declined, resulting in a smaller proportion of streamwater NO3− load removed over a given length of stream. Regional differences in stream denitrification rates were small relative to the proximate factors of NO3− concentration and ecosystem respiration rate, and land use was an important but indirect control on denitrification in streams, primarily via its effect on NO3− concentration.

VL - 54 UR - https://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.2009.54.3.0666 ER - TY - JOUR T1 - Stream denitrification across biomes and its response to anthropogenic nitrate loading JF - Nature Y1 - 2008 A1 - Mulholland, P.J. A1 - Helton, A.M. A1 - Poole, G.C. A1 - Hall, R.O. Jr. A1 - Hamilton, S.K. A1 - Peterson, B.J. A1 - Tank, J.L. A1 - L.R. Ashkenas A1 - Cooper, L.W. A1 - Dahm, C.N. A1 - W. K. Dodds A1 - Findlay, S.E.G. A1 - Gregory, S.V. A1 - Grimm, N.B. A1 - Johnson, S.L. A1 - W.H. McDowell A1 - Meyer, J.L. A1 - Valett, H.M. A1 - Webster, J.R. A1 - Arango, C.P. A1 - Beaulieu, J.J. A1 - Bernot, M.J. A1 - Burgin, A.J. A1 - Crenshaw, C. A1 - Johnson, L. A1 - Niederlehner, B.R. A1 - O'Brien, J.M. A1 - Potter, J.D. A1 - Sheibley, R.W. A1 - Sobota, D.J. A1 - Thomas, S.M. AB -

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing1, 2 and terrestrial ecosystems are becoming increasingly nitrogen-saturated3, causing more bioavailable nitrogen to enter groundwater and surface waters4, 5, 6. Large-scale nitrogen budgets show that an average of about 20–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins7, 8, indicating that substantial sinks for nitrogen must exist in the landscape9. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification6, 10, 11. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.

VL - 452 UR - https://www.nature.com/articles/nature06686 ER - TY - JOUR T1 - Factors affecting ammonium uptake in streams - an inter-biome perspective JF - Freshwater Biology Y1 - 2003 A1 - Webster, J.R. A1 - P. Mulholland A1 - Tank, J.L. A1 - Valett, H.M. A1 - W. K. Dodds A1 - Peterson, B.J. A1 - W.B. Bowden A1 - Dahm, C.N. A1 - S.E.G. Findlay A1 - Gregory, S.V. A1 - Grimm, N.B. A1 - Hamilton, S.K. A1 - Johnson, S.L. A1 - Marti, E. A1 - W.H. McDowell A1 - Meyer, J.L. A1 - Morrall, D.D. A1 - Thomas, S.A. A1 - Wollheim, W.M. AB - 1. The Lotic Intersite Nitrogen eXperiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter-biome variability of ammonium uptake to physical, chemical and biological processes. 2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole-stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of 15N-ammonium and downstream measurements of 15N-ammonium concentration. 3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in-stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake. 4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa). 5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake. VL - 48 ER - TY - JOUR T1 - Can uptake length in streams be determined by nutrient addition experiments? Results from an inter-biome comparison study JF - Journal of the North American Benthological Society Y1 - 2002 A1 - Mulholland, P.J. A1 - Tank, J.L. A1 - Webster, J.R. A1 - W.B. Bowden A1 - W. K. Dodds A1 - Gregory, S.V. A1 - Grimm, N.B. A1 - Hamilton, S.K. A1 - Johnson, S.L. A1 - Marti, E. A1 - W.H. McDowell A1 - Merriam, J. A1 - Meyer, J.L. A1 - Peterson, B.J. A1 - Valett, H.M. A1 - Wollheim, W.M. KW - ammonium KW - nitrogen limitation KW - nutrient cycling KW - nutrient spiraling KW - stream KW - uptake length AB - Nutrient uptake length is an important parameter for quantifying nutrient cycling in streams. Although nutrient tracer additions are the preferred method for measuring uptake length under ambient nutrient concentrations, short-term nutrient addition experiments have more frequently been used to estimate uptake length in streams. Theoretical analysis of the relationship between uptake length determined by nutrient addition experiments (SW′) and uptake length determined by tracer additions (SW) predicted that SW′ should be consistently longer than SW, and that the overestimate of uptake length by SW′ should be related to the level of nutrient addition above ambient concentrations and the degree of nutrient limitation. To test these predictions, we used data from an interbiome study of NH4+ uptake length in which 15NH4+ tracer and short-term NH4+ addition experiments were performed in 10 streams using a uniform experimental approach. The experimental results largely confirmed the theoretical predictions: SW′ was consistently longer than SW and SW′:SW ratios were directly related to the level of NH4+ addition and to indicators of N limitation. The experimentally derived SW′:SW ratios were used with the theoretical results to infer the N limitation status of each stream. Together, the theoretical and experimental results showed that tracer experiments should be used whenever possible to determine nutrient uptake length in streams. Nutrient addition experiments may be useful for comparing uptake lengths between different streams or different times in the same stream, however, provided that nutrient additions are kept as low as possible and of similar magnitude. VL - 21 ER - TY - JOUR T1 - N uptake as a function of concentration in streams JF - Journal of the North American Benthological Society Y1 - 2002 A1 - W. K. Dodds A1 - López, A.J. A1 - W.B. Bowden A1 - Gregory, S. A1 - Grimm, N.B. A1 - Hamilton, S.K. A1 - Hershey, A.E. A1 - Marti, E. A1 - W.B. McDowell A1 - Meyer, J.L. A1 - Morrall, D. A1 - Mulholland, P.J. A1 - Peterson, B.J. A1 - Tank, J.L. A1 - van der Hoek, D.C.J. A1 - Webster, J.R. A1 - Wollheim, W. KW - advective transport KW - ammonium KW - areal uptake KW - benthos KW - kinetics KW - mass transport KW - Nitrate KW - nitrogen AB - Detailed studies of stream N uptake were conducted in a prairie reach and gallery forest reach of Kings Creek on the Konza Prairie Biological Station. Nutrient uptake rates were measured with multiple short-term enrichments of NO3− and NH4+ at constant addition rates in the spring and summer of 1998. NH4+ uptake was also measured with 15N-NH4+ tracer additions and short-term unlabeled NH4+ additions at 12 stream sites across North America. Concurrent addition of a conservative tracer was used to account for dilution in all experiments. NH4+ uptake rate per unit area (Ut) was positively correlated to nutrient concentration across all sites (r2 = 0.41, log–log relationship). Relationships between concentration and Ut were used to determine whether the uptake was nonlinear (i.e., kinetic uptake primarily limited by the biotic capacity of microorganisms to accumulate nutrients) or linear (e.g., limited by mass transport into stream biofilms). In all systems, Ut was lower at ambient concentrations than at elevated concentrations. Extrapolation from uptake measured from a series of increasing enrichments could be used to estimate ambient Ut. Linear extrapolation of Ut assuming the relationship passes through the origin and rates measured at 1 elevated nutrient concentration underestimated ambient Ut by ∼3-fold. Uptake rates were saturated under some but not all conditions of enrichment; in some cases there was no saturation up to 50 μmol/L. The absolute concentration at which Ut was saturated in Kings Creek varied among reaches and nutrients. Uptake rates of NH4+ at ambient concentrations in all streams were higher than would be expected, assuming Ut does not saturate with increasing concentrations. At ambient nutrient concentrations in unpolluted streams, Ut is probably limited to some degree by the kinetic uptake capacity of stream biota. Mass transfer velocity from the water column is generally greater than would be expected given typical diffusion rates, underscoring the importance of advective transport. Given the short-term spikes in nutrient concentrations that can occur in streams (e.g., in response to storm events), Ut may not saturate, even at high concentrations. VL - 21 ER - TY - JOUR T1 - Control of nitrogen export from watersheds by headwater streams JF - Science Y1 - 2001 A1 - Peterson, B.J. A1 - Wollheim, W.M. A1 - Mulholland, P.J. A1 - Webster, J.R. A1 - Meyer, J.L. A1 - Tank, J.L. A1 - Marti, E. A1 - W.B. Bowden A1 - Valett, H.M. A1 - Hershey, A.E. A1 - W.H. McDowell A1 - W. K. Dodds A1 - Hamilton, S.K. A1 - Gregory, S. A1 - Morrall, D.D. AB - A comparative 15N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds. VL - 292 ER - TY - BOOK T1 - Stream Research in the LTER Network Y1 - 1993 A1 - Meyer, J.L. A1 - Crocker, T. A1 - D'Angelo, D. A1 - W. K. Dodds A1 - Findlay, S.E.G. A1 - Oswood, M. A1 - Repert, D. A1 - Toetz, D. PB - LTER Network LTER publication no. 15 CY - Seattle, WA ER -